Producing diphenyl



Patented Oct. 23, 1934 UNITED STATES PRODUCING DIPHENYL William H.Williams, Midland, Mich assignor to The Dow Chemical Company, Midland,Mich, a corporation of Michigan No Drawing. Application April 26, 1933,Serial No. 668,039

10 Claims.

The present invention is concerned with the production of diphenyl bypyrolysis of benzene, and has particular regardto preventinginterruptions of the process caused by the formation of s carbondeposits in the pyrolyzing apparatus.

In my prior applications, Serial No. 514,450, filed February 9, 1931,and Serial No. 648,654, filed December 23, 1932, I have described amethod and apparatus for pyrolyzing benzene in which the 10 benzenevapors are passed through electrically heated carbon or graphite tubularelements maintained at temperatures between 650 and 950 C. The vaporsare caused to flow at a velocity such that between about 5 and per centthereof are 16 pyrolyzed at a single pass, the conversion productsconsisting chiefly of diphenyl together with a small amountof higherboiling material of a tarry nature. The reaction products are con- Idensed and separated from the residual unreacted Ell-benzene, and thelatter is then recycled in the process. At the lower conversion ratesthe percentage: of diphenyl in the reaction product is highest, and asthe rate is increased the percentageof diphenyl decreases and that oftarry material increases, although the output of diphenyl may beincreased. In general it has been foundinadvisable to operate at aconversion rate much in excess of 20 per cent, due to excessive tarformation. v

. In the large scale practice of the process, however, operatingdiificulties are encountered which are due largely to the gradualformation of ad herent deposits ofcarbon in the tubular apparatus Theaccumulation of carbon gradually constricts the bore of the tubes,thereby disturbing the bal-; ancebetweengas fiow and electric current,and

eventuallymay cause stoppage of gas how if the obstruction is notremoved. Consequently periodical shutdowns have been necessary, for theIn addition to the carbon deposits, more or less loose or sooty carbonis also formed, which accompanies the reaction gases andcontaminates"the condensate obtained therefrom. Such 'car-.

.55 bon, however formed, represents a loss to' the process, which it isdesirable to prevent as far as possible. l

I have discovered that the formation of carbon is inhibited by thepresence of a small amount of sulphur compounds in thebenzene undergoingan pyrolysis. Commercially pure benzene at the present time commonlycontains about 0.20 per cent of sulphur in the form of organic sulphurcompounds. I have found the aforesaid pyrolyzing process can beconducted without material 5 formation of carbon when the raw materialconsists of such commercial benzene containing about 0.20 per cent orsulphur. However, the continuous operation of the process involves therecycling of from 95 to 80 per cent of thebenzene at 1 each pass. Suchrecycled benzene has a materially reduced sulphur content, becauseduring the pyrolysis a considerable portion of the sulphur compounds inthe benzene isdecomposed and escapes with the vent gases. Consequently Ihave foundit necessary,in order to maintain the process in continuousoperation without materialformation of carbon deposits, toadd a smallamount of sulphur or volatile sulphur compound to the benzene at eachcycle sufficient to com- 3 pensate for that lost in the vent gases. Theinvention, then, consists in the improved method hereinafter fullydescribed and particularly pointed out in the claims. r

When the pyrolysis of benzene bythe aforesaid 5 process is conducted atsuch rate that the percentage of benzene converted at each pass isabout15 per cent by weight, for example, the benzene charge entering thepyrolyzing furnace contains about 15 per cent of fresh benzene and 0'per cent of recycled benzene; The sulphur content of the fresh benzenemay be about 0.20 per cent and that of the recycled benzene about 0.14per cent, hence the mixture would have a' sulphur content of about 0.15per cent. The sul- 5 phur content of the benzene would be rapidlydepleted by the loss thereof occurring in each succeeding cycle of theprocess, until too little sulphur'remained to be efiective forpreventing to the benzene may be made at any convenient point in theprocess cycle. For instance, it may be added directly to the recycledbenzene, or along with the fresh benzene introduced as make- 5 up ineach cycle. The proportion of sulphur or sulphur compound used will varyaccording to operating conditions, but will be regulated to maintain theaverage sulphur content of the benzene pyrolyzed above the pointnecessary to prevent any material deposition of carbon.

When operating the process with maintenance of the sulphur content ofthe benzene between about 0.15 and 0.25 per cent by suitable additionsof sulphur compounds to the benzene, I

have been enabled to maintain continuous operation for a period ofseveral weeks without shutdown, and at the end of the period nosubstantial accumulation of carbon was found in the apparatus. On theother hand, when pyrolyzing benzene largely free from sulphur, the

formation of carbon deposits necessitated interrupting the process toclean out carbon at intervals varying from several hours to a few days,at most.

a The addition of sulphur itself, either in vapor or powdered form, isnot always convenient, owing to the much lower vapor pressure of sulphuras compared to benzene, and the use of a gaseous or easily volatilizablesulphur compound is simpler and easier to carry out. Examples ofsuitable sulphur compounds are sulphur dioxide,

hydrogen sulphide, carbon bisulphide, mercaptans, thiophenols,thiophene, etc. In practice I prefer to use carbon bisulphide, which onaccount of its volatility can be introduced simply by feeding aregulated quantity of the liquid directly into the stream of benzenevapors before the latter reach the pyrolyzing zone.

The pyrolyzing process is carried out as de- 40 a scribed in the priorapplications already referred to, by vaporizing benzene and passing thevapors into a reactor, wherein they are pyrolyzed by contact withelectrically heated carbon surfaces maintained at a temperature above650 C. but

; not exceeding 950 C. The reactor in its preferred "form contains oneor more tubes of carbon or graphite with electrical connections thereto.The benzene vapors are preheated by passing in contact with the outsideof the electrically heated s carbon tubes and then passed through thetubes 'wherein the temperature is raised to the point required for theformation of diphenyl. The reaction vapors are cooled and condensed toseparate the readily liquefiable products from the s 5anon-condensablegases, which latter are vented off. The condensed material is distilledto separate unreacted benzene from diphenyl and other high boilingmaterials, such benzenebeing returned to the pyrolysis step afteradmixture with fresh benzene to make up for the amount converted todiphenyl in the previous cycle. The

carbon bisulphide or equivalent sulphur compound may conveniently beintroduced along with the fresh benzene, a sufficient amount of thesulphur compound being added to raise the sulphur content of the mixedbenzene vapors to the desired point to prevent carbon deposition in thereactor. A further advantage of operating in the manner described isthat metallic surfaces of the reactor which come in contact with the hotbenzene vapors may be made of usual structural materials suitable foruse at the temperatures employed, thus avoiding all necessity for'usingspecial metals or alloys, or to provide surfaces with specially preparedprotective coatings for counteracting the formation of carbon deposits,as has been previously proposed.

Although the invention has been particularly described as applied to theprocess wherein benzene is pyrolyzed by contacting with electricallyheated carbon or graphitized carbon surfaces, nevertheless it is notlimited thereto. It is also applicable to any process for producingdiphenyl wherein benzene is subjected to a temperature between 650 and950 C. in a pyrolyzing zone, irrespective of the 'mode of heatingemployed, whether electrical, by means of an externally heated retort,or by passing the benzene vapors through a bath of molten metal or salt.

Other modes of applying the principle of my invention may be employedinstead of the one explained, change being made as regards the processherein disclosed, provided the step or steps stated by any of thefollowing claims or the equivalent of such stated step or steps beemployed.

, I therefore particularly point out and distinctly claim as myinvention:-

1. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C., the improvement whichconsists in maintaining in the benzene a sulphur content in excess ofabout 0.10 per cent by adding a substance from the class consisting ofsulphur and volatile sulphur compounds to the benzene before it entersthe pyrolyzing zone.

2. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C., the improvement whichconsists in maintaining in the benzene a sulphur content in excess ofabout 0.10 per cent by adding carbon bisulphide to the benzene before itenters the pyrolyzing zone. I

3. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C. and unreacted benzene isseparated from the reaction product and recycled in the pyrolyzing step,the improvement which consists in adding a substance from the classconsisting of sulphur and volatile sulphur compounds to the recycledbenzene in amount sufiicient to maintain the sulphur content thereof inexcess of about 0.10 per cent.

4. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C. and unreacted benzene isseparated from the reaction product and recycled in the pyrolyzing step,the improvement which consists in adding a substance from the classconsisting of sulphur and volatile sulphur compounds to the recycledbenzene in amount sufficient to maintain the sulphur content thereofbetween about 0.15 and about 0.50 per cent.

5. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C. and unreacted benzene isseparated from the reaction product and recycled in the pyrolyzing step,the improvement ature between 650 and 950? C. and unreacted benzene isseparated fromthe reaction product and recycled in the pyrolyzing step,the improvement which consists in adding carbon bisulphide to therecycled benzene in amount suflicient to maintain the sulphur contentthereof between about 0.15 and about 0.50 per cent.

7. In a process of producing diphenyl, wherein benzene is pyrolyzed byheating at a temperature between 650 and 950 C. and unreacted benzene isseparated from the reaction product and recycled in the pyrolyzing step,the improvement which consists in mixing such recycled benzene with acomponent of fresh benzene and with a suflicient amount of carbonbisulphide to produce in the benzene mixture a sulphur content betweenabout 0.15 and 0.50 per cent and and higher boiling reaction products,adding a substance from the class consisting of sulphur and volatilesulphur compounds to such separated benzene in amount suflicient toproduce a sulphur content in excess of 0.10 per cent therein, andreturning such sulphur-containing benzene to the pyrolysis step.

9. The process of producing diphenyl which comprises passing benzenethrough a pyrolyzing zone in contact with electrically heated carbonsurfaces at a, temperature between 650 and 950 C., separating unreactedbenzene from diphenyl and higher boiling reaction products, adding tosuch separated benzene a component of fresh benzene and a sufiicientamount of a substance from the class consisting of sulphur and volatilesulphur compounds to produce a sulphur content between about 0.15 and0.50 per cent in the benzene mixture, and introducing said mixture intothe pyrolyzing zone.

10. The process of producing diphenyl which comprises passing benzenethrough a pyrolyzing zone in contact with electrically heated carbonsurfaces at a temperature between 650 and 950 C., separating unreactedbenzene from diphenyl and higher boiling reaction products, adding tosuch separated benzene a component of fresh benzene and a sufficientamount of carbon bisulphide to produce a sulphur content between about0.15 and 0.50 per cent in the benzene mixture, and introducing saidmixture into the pyrolyzing zone.

WILLIAM H. WILLIAMS.

